The subject matter of the present invention relates to search loops for metal detectors and, in particular, to balanced search loops in which the magnetic flux produced by the transmit coil is balanced by the magnetic flux of an auxiliary coil so that the receive coil output voltage is nulled to substantially zero when no metal object is present and the loop is located in air remote from the ground. Previously, balanced search loops suffered from the defect that changes in temperature or mechanical stresses caused these coils to move relative to one another and produce a false output signal on the receive coil because the loop was no longer balanced. This problem is overcome in the balanced search loop of the present invention by supporting a magnetic flux feedback coil directly on the receive coil. The flux of the feedback coil cancels the flux of the transmit coil to provide a zero or balanced flux region containing the receive coil when no metal object is present. The present search loop has the additional advantage of providing a balanced flux region of a large area bounded by the receive coil which does not have to be exactly concentric with the transmit coil so the loop is less expensive to manufacture while also giving high sensitivity and good pinpointing of metal targets.
Previously it has been proposed to provide balanced search loops for metal detectors in plurality of different shapes and coil arrangements. For example, see the crossing coils, the coaxial coils, and the coplanar concentric coils shown in the book "Successful Treasure Hunting" by Charles Garrett published in 1974 and revised edition in 1978, pages 171 to 173 and 189 to 192. However, all of these prior loop designs suffer from the defect that during operation of the metal detector there is some relative movement of the coils with respect to one another due to temperature changes and/or mechanical stress on the coils. Since for sensitivity reasons the receive coil has many more turns than the transmit coil, there is a transformer step-up action which makes the receive coil output signal very sensitive to change in the relative position of the coils. In addition, in many designs the receive coil crosses the transmit coil or auxiliary coil used for balancing which means that with relative movement the overlapping area of the two coils increases in one portion and decreases in another portion of such area. This double effect makes the loop even more sensitive to relative movement due to temperature changes or mechanical stress. Furthermore, in many of these designs the magnetic field of the transmit coil is a complex pattern which results in nonuniformity of pickup sensitivity. As a result, maximum sensitivity is many times not in the geometric center of the loop; in addition, objects closer to the outside of the loop may generate signals of different phase from those generated by the same object at a farther distance from the loop. These phase differences cause errors in interpreting the response data of detectors since it causes the receive signals of conductive and magnetic targets to be reversed from their normal response.
The above problems are overcome in the balanced search loop of the present invention by supporting the auxiliary coil or magnetic flux feedback coil directly on the receive coil such as by wrapping the turns of the feedback coil over the outside turns of such receive coil. This makes the feedback coil concentric with the receive coil and causes substantially all of the magnetic flux from the feedback coil to intercept the receive coil. Since the feedback coil is supported on the receive coil, thermal expansion and mechanical stresses affect both of the coils equally causing no appreciable relative movement between the two coils. This means that the magnetic flux of the feedback coil can cancel the magnetic flux of the transmit coil which intercepts the receive coil to balance the loop at all times so that the output of the receive coil is substantially zero under a nulled condition in air and remote from the ground when no metal object is present. A larger region of balance is provided in the present loop which extends over substantially the entire area of the receive coil. Furthermore, the present loop is capable of high sensitivity and good pinpointing for locating the targets' position more exactly.
It is a simple matter by choosing the proper turns ratio between the feedback coil and the transmit coil to cause their magnetic flux to balance in the region bounded by the receive coil. This turns ratio is substantially equal to the ratio of the areas bounded by the feedback coil and the transmit coil. Fine adjustment or nulling is accomplished by movement of a partial coil turn connected between the transmit coil and feedback coil. It should be noted that the balance region is actually slightly larger than the receive coil which enables the receive coil and feedback coil to be more easily positioned correctly within the transmit coil during manufacture because they do not necessarily have to be exactly concentric with the transmit coil. In addition, there is no need for expensive "potting" of coils in epoxy resin since there can be some slight movement of the self-supported unit formed by the feedback coil and the receive coil relative to the transmit coil without causing erroneous receive signals.